Doh. In that case, ignore the code in my response. You might be able to use a plugin attached to OnDocFormPrerender. I'm not sure it would work and if the Resource gets saved, the Setting value would replace the tag.
I guess you could also do it with an @EVAL TV, but it wouldn't be pretty.
The code would look something like my code above, but with "||" and "==" tokens in the output.
Maybe something like this (all on one line):
@EVAL $resources = $modx->getCollection('modResource', array('parent' => $modx->getOption('shop')));
foreach($resources as $resource) {
temp[]= $resource->get('pagetitle') . '==' . $resource->get('id');
}
return implode('||', $temp;